Perfluoroalkylpolyethers are of current interest for many new material applications due to their lack of chemical reactivity and their outstanding thermal stability. Their remarkable stability, comparable to that of perfluoroalkanes, along with their interesting surface properties, viscosities and broad liquid ranges make saturated perfluoropolyethers attractive solvents, hydraulic fluids, heat transfer fluids, vacuum pump oils, lubricants, and grease base stocks. Very high molecular weight perfluoropolyether solids have potential uses as sealants, elastomers, and plastics. See Paciorek, K. J. L, Kaufman, J., Nakahara, A., Journal of Fluorine Chemistry, 10, 277 (1977); McGrew, F. C., Chemical Engineering News, 45, 18 (August 7, 1967); Eleuterio, H. S., Journal of Macromolecular Science-Chemistry, A6, 1027 (1972).
Several synthetic methods exist for preparing saturated perfluoropolyethers. The anionic polymerization of perfluoroepoxides, particularly hexafluoropropylene oxide and tetrafluoroethylene oxide, have been used with success. See Hill, J. T., Journal of Macromolecular Science-Chemistry, A6, 1027 (1972). The preparation of perfluoropolyethers via this method first involves the oxidation of a perfluoroolefin to a perfluoroepoxide, followed by an ionic polymerization of the epoxide to an acyl fluoride terminated perfluoropolyether and conversion of the acyl fluoride end-groups to unreactive end-groups by decarboxylation reactions. The inability to form very high molecular weight polymers, the lack of stability of many perfluoroepoxides, and the extreme difficulty encountered when attempting to polymerize substituted perfluoroepoxide have been cited as drawbacks associated with this art. Additionally, anionic polymerization of perfluoroepoxide does not lend itself well to the manufacturing of perfluoro copolymers since perfluoroepoxide vary widely in reactivity.
An alternative synthetic method for the production of perfluoropolyethers involves the UV photolysis of tetrafluoroethylene and/or hexafluoropropylene in an inert solvent in the presence of oxygen. This multistep process yields an acyl fluoride terminated polymer containing both the --CF.sub.2 --, --CF.sub.2 --CF.sub.2 --CF.sub.2 --CF.sub.2 --, (CF.sub.2 --CF.sub.2 --O), and (CF(CF.sub.3)--CF.sub.2 --O) repeating units as well as unstable peroxidic oxygen linkages (CF.sub.2 --O--O--CF.sub.2). Treatment of the polymer at elevated temperatures and with fluorine gas gives a stable polymer containing perfluoroalkyl ends groups. See U.S. Pat. Nos. 3,665,041; 3,847,978; 3,770,792; and 3,715,378.
Although this process can be used successfully to prepare copolymers, the process is completely random with little control of the kinds and numbers of repeating units. Undesirable linkages such as the peroxidic oxygen and the poly(difluoromethylene) portions of the polymer are unavoidable and give the polymer undesirable properties for many applications. The formation of by-product polytetrafluoroethylene and the need for fairly exotic solvents adds significantly to the production costs of the polymer.